Personally i would prefer Shanks as the final villain.
this would make a bigger impact with Luffy's reunion with Shanks. Luffy's last opponent was the guy who inspired him.
It may be good for me but if Shanks keep being the way he is, Shanks being the final boss will make Zoro and Mihawk supporters celebrate in a way thats too toxic imo.
"Even Luffy final opponent is a canonically inferior version of Zoro's final opponent" lol i can already imagine the toxicity, circle jerking and echo chambers.
Unless Shanks let go of his sword and devour Imu or something, obtaining an absurd power or revealing a sword-free fighting style, he will always be recognized by Zoro/Mihawk fandom as only swordsman and nothing more. Just like how even Roger was considered canonically weaker than Mihawk by many Zoro fandom.
So yeah tldr Shanks being the final boss will make this forum 10 times more toxic due to Shanks vs Mihawk and Luffy vs Zoro eternal dick measuring.